Cathode ray tubes



April 9, 1957 LENG CATHODE RAY TUBES Filed Aug. 4, 1955 o o A-r-roaui s United States CATHODE RAY TUBES Geoifrey Leng, Great Baddow, England, assignor to Marconis Wireless Telegraph Company Limited, London, England, a British company Application August 4, 1955, Serial No. 526,395

Claims priority, application Great Britain August 4, 1954 4 Claims. (Cl. 315-22) This invention relates to cathode ray tubes and though not limited to its application thereto is primarily intended for, and of particular advantage in connection with, relatively large cathode ray tubes wherein a relatively short neck portion merges into a tapered portion (usually, though not necessarily, conical) of wide angle and terminating in a relatively flat end wall.

Modern tendencies in the design of cathode ray tubes for such purposes as television reproduction are towards the use of increasingly wide angle tapered portions and relatively flat end walls. With such tubes considerable difficulties are encountered in obtaining good focussing of the cathode ray in all positions of deflection.

The invention is illustrated in and explained in connection with the accompanying drawing which shows diagrammatically one embodiment of the invention so far as is necessary to an understanding thereof.

Consider the tube represented in broken away elevation in the drawing. This tube has a neck portion 1, a conical portion 2 and an end wall 3. The deflection centre of the cathode ray in the tube is represented by a cross 4 and a typical ray position by a chain line 5. It will be seen that the end wall 3 is relatively flat and does not follow the spherical surface which it would have if it were arcuate with the deflection centre 4 as centre. This spherical surface is represented by the broken line 6.

A screen is deposited as in the usual way on the inside of the end wall 3. If the ray is correctly focussed on the screen when in the undeflected position, i. e. when it impinges centrally on the screen, it will become increasingly out of focus on the screen as deflection away from the central position increases for the locus of the points for which the beam will be correctly focussed lies on the spherical surface 6. Focussing error arising in this way is obviously most serious in wide angled tubes. Though such focussing error can be reduced or eliminated by using what may be termed a focus modulating electromagnet whose energisation is controlled with deflection so as to alter the distance of the focus from the deflection centre as deflection takes place, known arrangements of this nature are not only complex but fail to satisfy certain operating requirements now to be described.

It is a general requirement for the achievement of economic design in the line deflection circuits of a television or similar cathode ray tube that the Q value of the line deflector coil shall be as high as possible. In any tube with focussing and deflection coil assemblies round its neck the focussing coil is bound to be near the deflecting coil assembly, particularly in a tube with a wide angle and a short neck, in order that the size of the spot on the screen shall be small and the resolution accordingly high. If, however, the above mentioned high Q value is to be obtained the focussing coil assembly must not, despite its closeness to the deflecting coil assembly, introduce substantial losses into or otherwise undesirably interfere with the said deflecting coil assembly, e. g. by magnetic couping therewith or by the introduction of eddy current losses therein. Satisfaction of this requirement is there- 2,788,468 Patented Apr. 9, 1957 fore a very diflicult matter and largely for this reason usual present day practice is to use a fixed (i. e. unmodulated) ray focussing system and accept the resultant loss of accuracy of focus during deflection.

According to this invention a cathode ray tube is provided with a focussing system including two permanent magnets arranged adjacent one another and in magnetic opposition along the neck of the tube and two focus modulating coils also connected in magnetic opposition to each other, one of said coils being arranged when energised to oppose the field of one magnet and the other arranged when energised to oppose the field of the other magnet, said coils being in use energised in dependence upon ray deflection so that as the ray is deflected the combined focussing effect due to the magnets and the coils maintains substantially correct focussing in all positions of the ray.

Preferably the magnets are ring magnets arranged side by side along the neck of the tube and preferably also one of the coils is mounted in one ring magnet and the other inside the other. Again preferably the longitudinal spacing between the magnets is adjustable for the purpose of adjusting the unmodulated focussing component, i. e. the component which provides accurate focussing when the coils are not energised and the ray is central in the tube.

Referring again to the accompanying drawing there are provided two permanent magnets 7, 8 preferably of the magnetic material known under the trade designation Magnadur. These magnets encircle the neck 1 of the tube and are mounted side by side in magnetic opposition. The spacing between them is arranged to be adjustable by any convenient means (not shown) to facilitate accurate focussing of the ray 5 by the magnets alone when the ray is in its undeflected position. Inside each of the two magnets is a focus modulating coil 9 or 10. These coils are connected in series and magnetic opposition, the field of the coil 9 when energised opposing that of the magnet 7 and the field of the coil 10 when energised opposing that of the magnet 8. The series connected coils are energised in any convenient manner known per se in dependence upon deflection of the ray. Thus they may be energised through an amplitude control unit 11 from the normally provided ray deflection circuits 12. So as not to complicate the drawing the ray deflecting coil assembly is not represented but it is, of course, provided as in the usual way.

The illustrated arrangement has important practical advantages as compared to previously known proposals. The permeability of magnetised Magnadurthe preferred magnetic material-4s near unity and consequently the use of this material in association with the focussing coils only slightly increases their inductance and therefore keeps the required reactive power low while in addition, because the magnets have near unity permeability, they do not appreciably distort the field of the deflecting coils. Moreover, since Magnadur behaves like an insulator the lens can be placed very close to the deflector coils without appreciable damping eifects due to eddy currents and the further the lens can be removed from the cathode and the nearer it can be brought to the screen, the smaller is the spot size, the less the image magnification and the smaller the lens power required for a given aperture. The smaller the spot size the better the resolution and the smaller the lens power required the smaller the reactive power required in the coils and therefore the smaller and cheaper are the control circuits required. The dual arrangement of focussing coil described gives very little external field and, in consequence, spiral distortion of the raster is very small. Such distortion is apt to be very serious if a single coil lens system is placed very close to the deflector coils because the lens field leaks into the deflector'coil'region and when the beam is deflected by even a small amount inside the deflector coils and it passes through even a weak focussing field spiral distortion is produced. The arrangement of the focussing coils close to the neck of the tube and inside the magnet rings means that the coil diameters are small and this too leads to l0w coil;inductance and consequent low reactive power required.

I claim:

1. A cathoderay tube focussing system comprising two permanent magnets arranged adjacent one another and in magnetic-opposition along the neck of the tube and two-focus modulating coils also connected in magnetic opposition to each other, one of said coils being arranged-When energised .to oppose thefield of; one magnet and the other arranged when energised to oppose the ,field of the other magnet, gsaid ,coils being energised in dependence upon raydeflectionso that as the ray is deflected the combined'sfocussing effect due to the magnets and the coils maintains substantially correct focussing in all positions of the ray.

2. A system as claimed in claim 1 wherein the magnets are ring magnets arranged side by side along the neck of the tube with one coil mounted inside one ring magnet and the other coil mounted inside the other.

3. A system as claimed in claim 1 wherein the longitudinal spacing of the magnets is adjustable.

4. A system as claimed in claim 2 wherein the longitudinal spacing of the magnets is adjustable.

References Cited in the file ;o,f ,this patent UNITED STATES PATENTS 2,539,492 Symth Jan. 30, 1951 2,656,486 Giordano Oct. 20, 1953 2,724,075 Van Urk et a1 Nov. 15, 1955 2,730,642 Grosjean Jan. 10, 1956 2,740,064 Birkbeck Mar. 27,1956 

